A transistor has been used in a wide range of fields such as an electronic circuit of a computer or the like, or an IC of a thin film transistor (TFT) and Radio Frequency Identification (RFID).
In general, a transistor is configured by including a gate electrode, an insulating layer, a semiconductor layer, a source electrode, and a drain electrode on a substrate, and recently, an organic transistor including a semiconductor layer (an organic semiconductor layer) formed of an organic material has been developed. The organic transistor can be manufactured through steps which are simplified by a flexible circuit by using the characteristics of the organic material in that the organic material has flexibility and can be laminated by various printing methods, various coating methods, or the like which can be applied under the atmosphere rather than under vacuum, and thus has high usefulness.
One of the important indices for comparing the performance of transistors includes the mobility of a carrier. In the transistor, when a voltage is applied to the gate electrode, carriers are generated in the vicinity of a boundary surface between the semiconductor layer and the insulating layer, and electrical conductivity increases, and when a voltage is applied between the source electrode and the drain electrode, the carriers are moved to the source electrode or the drain electrode, and a current flows. At this time, when the mobility of the carrier is high, in the transistor, an operation from ON to OFF or an operation from OFF to ON is performed quickly, and a current value also increases. Therefore, preferred properties of the carrier are high mobility.
On the other hand, in a bottom gate type transistor including a gate electrode on the substrate side (a lower side) from the semiconductor layer, when surface smoothness of the insulating layer is low (concavities and convexities are large), carriers are moved along the boundary surface in the vicinity of the boundary surface between the semiconductor layer and the insulating layer, and the actual movement distance of the carrier is affected by the concavities and convexities on the surface of the insulating layer. Therefore, the actual movement distance becomes longer than in a case where the concavities and convexities are small (the smoothness is high), and thus the movement speed of the carrier, that is, the mobility decreases. Therefore, in such a transistor, it is important to increase the surface smoothness of the insulating layer.
In response, a method is disclosed in which a coated film is formed by using a photocurable resin, and then an insulating layer is formed by curing the resin due to light irradiation in a state where a glass plate or the like having high surface smoothness is placed on the coated film, and the glass plate or the like is removed, as a method for increasing the surface smoothness of the insulating layer (refer to PTL 1). In addition, a method is disclosed in which a smoothing layer is formed on an insulating layer, and an organic semiconductor layer is formed on the smoothing layer, as a method for lessening an influence of the smoothness of the insulating layer surface (refer to PTL 2).
However, in the method disclosed in PTL 1, it is necessary to arrange and remove the glass plate or the like, and in the method disclosed in PTL 2, it is necessary to form the smoothing layer, and thus in both of the methods, there is a problem in that a manufacturing step of the transistor is complicated.
Originally, the surface smoothness of the insulating layer is affected by the surface of the gate electrode which is arranged on the insulating layer to be in contact with the insulating layer, and when the surface smoothness of the gate electrode is low (the concavities and convexities are large), the surface smoothness of the insulating layer also decreases according to the surface smoothness of the gate electrode. In response, neither of the methods disclosed in PTL 1 and PTL 2 can fundamentally solve the problem at the time of forming the gate electrode through the simplified steps. This is because, when the gate electrode is formed by a method such as vacuum deposition or sputtering, it is possible to increase the surface smoothness, but since a special device is required, the steps become complicated, and the cost increases, and when the gate electrode is formed by various printing methods, various coating methods, or the like, as in the semiconductor layer described above, the gate electrode is formed under the atmosphere, and the gate electrode can be manufactured at a lower cost by more simplified steps, but the surface smoothness decreases.